Measurement apparatuses for three-dimensional shapes are used in an area generally referred to as robot vision, such as three-dimensional shape inspection of products, measurement of physical dimensions, assembly positioning.
There are various methods for non-contact measurement of three-dimensional shapes, and these methods have advantages and disadvantages. For example, a light section method using laser slit light, which is a method for measurement of three-dimensional shapes, has an advantage in that it can be carried out using compact components such as a scanner, a mirror, and a semiconductor laser. However, the light section method has a disadvantage in that this method requires capturing as many images as there are scan lines and, hence, is unfavorable for high-speed measurement of an object having a three-dimensional shape.
As a method to overcome such a drawback, there is a method called a space encoding method in which a three-dimensional shape is measured by projecting a slit-shaped light pattern, which is a light and dark pattern, onto an object, for space division. In the space encoding method, the number of images to be captured needs to be only (log2 N), where N is the number of divisions of space. Hence, compared with the light section method, the space encoding method allows an object with a three-dimensional shape to be measured using fewer captured images.
However, in the space encoding method, the resolution of space division is limited by the number of bits used for quantization of a projected pattern, at the time of space division.
Hence, there is a method called a multi-slit-light space measurement method in which a space encoding method is used up to a certain stage and the three-dimensional shapes of individual spatially divided regions are measured by a light section method at the same time (refer to PTL 1).
This method allows an object having a three-dimensional shape to be measured with even fewer captured images than in the light section method, and allows an object having a three-dimensional shape to be measured with higher spatial resolution than measurement using only the space encoding method.
Hitherto, the accuracy with which the three-dimensional shape of an object is measured has been improved using the principles described above.
A three-dimensional shape measurement apparatus, which measures a shape by, for example, projecting laser slit-shaped light or a slit-shaped light pattern, has a problem in, for example, detection of measurement lines on which measured values are based, depending on the capturing condition of the reflected light of projected light. In other words, detection of measurement lines may not be performed under ideal condition (diffuse reflection) and there may be specularity, depending on a bidirectional reflectance distribution function (BRDF), which expresses the reflection characteristics of light from an object. Due to this, for example, in a three-dimensional shape measurement apparatus, indirect reflection of measurement lines is measured.
In such a case, since real measurement lines (measurement lines due to direct reflection) and false measurement lines (measurement lines due to indirect reflection) are observed, the reliability of measured distance values may become low. Further, when the reflection strength of an object is low, the signal to noise ratio (S/N ratio) of measurement lines may decrease, resulting in difficulty in the observation of the object and decreased reliability of the measured distance values when numerous noise components are observed, for example.
In addition, a problem arises that depends on the state of inclination of an object when the object is viewed from the observation direction of the object. When the surface of an object is perpendicular to the light axis of the projector apparatus and camera apparatus of a three-dimensional measurement apparatus, this is considered to be a good condition as a measurement state. However, when the angle between the surface of the object and the light axis of the projector apparatus and camera apparatus of a three-dimensional measurement apparatus is changed from the state described above due to inclination, the amount of reflected light due to projection of laser slit light or a slit-shaped light pattern decreases. This results in increased noise in the observed signal and a decreased signal to noise ratio.
Further, when a distance between measurement lines becomes smaller than the pixel resolution of a camera apparatus, that is, when the measurement resolution of a slit-shaped light pattern exceeds the Nyquist frequency of the resolution of the camera apparatus, the state of measurement becomes unstable near the corresponding portion of an image.
Hence, in PTL 1, encoding errors are detected by projecting, onto an object, a light pattern for detecting an encoding error of a code indicating space division of a portion of a slit-shaped light pattern.
However, the method disclosed in PTL 1 requires, for example, projection of a new pattern to remove the influence of indirect light. This results in an increase in the number of operations of projecting a slit-shaped light pattern, although the influence of indirect light can be measured. In other words, the method disclosed in PTL 1 requires capturing images more frequently than in ordinary methods in measurement of three-dimensional shapes. Hence, the method disclosed in PTL 1 causes an increase in measurement time and makes high-speed measurement of the three-dimensional shape of an object difficult.
Further, there is a case in which a slit-shaped light pattern that has a frequency higher than the Nyquist frequency is captured due to an inclination of the light axis of a camera apparatus and a projector apparatus with respect to the surface of an object as described above, or a case in which the reflectance of an object is low. It is not easy in the method disclosed in PTL 1 to cope with a decrease in the signal to noise ratio occurring in such cases.